The present invention relates to a pattern area ratio measuring apparatus for measuring an area ratio of a pattern drawn on a plate or lithographic printing plate to be measured.
A conventional pattern area ratio measuring apparatus of this type is comprised of a plurality of parallel light sources and radiates light from these light sources onto a plate to be measured, thereby measuring an area ratio of a pattern drawn on the plate to be measured on the basis of light reflected by the plate to be measured.
FIGS. 9A and 9B show a structure of a conventional pattern area ratio measuring apparatus. Referring to FIGS. 9A and 9B, reference numeral 1 denotes a plate to be measured; 2-1 to 2-n, light sources (halogen lamps) arranged parallel to each other in the widthwise direction of the plate 1 to be measured with intervals of several cm therebetween; and 3, a CCD camera.
In the pattern area ratio measuring apparatus having the above arrangement, light from the halogen lamps 2-1 to 2-n is radiated onto the plate 1 to be measured, and light reflected by the plate 1 to be measured is incident on the CCD camera 3. An area ratio of a pattern drawn on the plate 1 to be measured is measured on the basis of an output voltage from the CCD camera 3 generated in correspondence with the input reflected light.
The above conventional pattern area ratio measuring apparatus can measure a pattern area ratio without any trouble provided that a presensitized plate (to be referred to as a PS plate hereinafter) is used as the plate 1 to be measured. If, however, a waterless plate is used as the plate 1 to be measured, a pattern area ratio cannot be correctly measured by this apparatus.
That is, since the surface of a PS plate is grained, light is irregularly reflected and uniformized by the surface, and this uniformized light is incident as reflected light on the CCD camera 3. Therefore, a pattern area ratio of such a PS plate can be correctly measured. However, since the surface of a waterless plate is mirror-finished, its reflectivity is much higher than that of the PS plate. For this reason, the intensity of light reflected by a portion around an intersection with respect to each of optical axes L.sub.1 to L.sub.n of the halogen lamps 2-1 to 2-n is increased, and an output voltage value from the CCD camera 3 corresponding to the portion exceeds an upper voltage limit V.sub.UP, as shown in FIG. 10. As a result, a pattern area ratio different from an actual pattern area ratio is measured.